Regional Tectonic Settings
1. Formation of Bengal Basin
Bengal basin, busying a major part of Bangladesh, lies about between 25O-20O30’N latitudes and 87O30’-90O30’E longitudes. It comprises the full lowland of about 89000 sq kilometer among which 57000 sq kilometers are on land and 32000 sq kilometers are of offshore up to 200m bathemetry ( M. jain and M. Rastogi ) . The Bengal basin is bordered by the Chhota Nagpur tableland and Rajmahal Hills in West, Himalayan foothills in north, Meghalaya Plateau in the nor’-east, the Manipur-Tripura hills in the E and by Feni river at the sou’-east. The southern Boundary of this basin is marked by Bay of Bangal where the basin surface slopes into the Bengal deep sea fan. ( Banglapedia )
The formation of Bengal Basin began about 125 million old ages ago, at early third period as a divergent fringy basin in Indian home base drifting towards Laurasia. Before Cretaceus period the earth-landforms were divided into two home bases – Laurasia and Gondwana. Laurasia consisted of largely North American and European-Asian land mass. On the other manus Gondwanaland consisted of Indian home base including Bangladesh ( Greater Rangpur and Dinajpur country ) , South America, Australia, Africa and Antarctica ( Haroun er Rashid, 1991 ) . In early Third period the Indian home base, splited off from the Gondwanaland and moved comparatively fast towards Eurasiatic home bases. Subsequently about 45million old ages ago in mid-Eocene, the floating Indian home base collided with Eurasiatic home base in north and with Burmese home base in nor’-east ensuing subduction of Indian home base in both instance. The result was the disappearing of Tethys Sea and upliftment of the Himalaya scope and the big Tibetan tableland every bit good as a really considerable geological fault in China ( Mattauer et. Al. 1981 ) . After this hit, the position of Bengal basin converted as convergent fringy basin.
The eastern portion of the Bengal basin was built at Oligocene period as the northeasterly portion of India broke and sank into under the sea and reformed as portion of Bengal basin. The older stones of Gondwanaland surfaced and formed the Meghalaya tableland in E and Chhota Nagpur tableland in West. The country between these tablelands are called the Rajmahal spread and along this spread formed the land surface of Dinajpur, Naogaon and Rangpur of Bangladesh. ( Haroun er Rashid, 1991 ) .
2. Ganges Brahmaputra Delta
Beginning and Development of The Sundarbans
The beginning of the Sundarbans ways back to the late Pleistocene period. At that clip, the formation of delta in west Calcatta was in advancement by the deposit of Ganges river. With the transition of clip the flow of Ganges river migrated eastward and the formation of delta besides shifted towards present Jessor country.
By the clip the delta formation extended towards present Sundarban, the delta of east greater Barisal part was built by early Meghna and Brahmaputra rivers.
Subsequently in 15Thursdayor 16th century the class of Ganges river once more changed class at due east and led many left bank channels bear downing every bit right bank distributaries. A major left bank tributary, Teesta confluenced with Brahmaputra moved westward to run into with Ganges river in Goalando and formed the Padma river by 1830. Due to the complicated drainage form formed by incorporate rivers, channels, brooks, estuaries, deltaic landforms of assorted form, size and lift was in built. The Kumar, Chitra, Kabadak, Nabaganga, bhairab rivers became major distributaries of Ganges river and spreaded out all over Sundarban through their legion distributaries of each.
Traveling through all these pandemonium of river class shifting, channels and land forming, the present position of Sundarban came to existence about 4000 old ages ago. ( Rob and Ashaduzzaman, 2009 )
Geomorphologic Unit of measurements of the Sundarbans:
The Sundarbans is loosely considered as a Rhizophora mangle swamp with a complex combination of tidal creaks, river channels, estuaries, tidal flats, clay flats, sand dunes, lasting, semi-permanent swamps, beaches etc. Most outstanding landforms are presented in brief:
Alluvial Plains:
Alluvial fields form through long term deposition of river deposits, largely by fluvial procedures. It is extended upto the tidal plains covering the fresh water zone of river Ganges in Sundarban.
Tidal Flats or Mud flats:
Tidal flats or Mud flats are the deposition of estuarine silts, clays and carnal debris at the intertidal zone. These countries are barely vegetated and organize the in-between portion of tidal basin, below the flora back uping salt fen.
Salt Marsh:
Salt fens are landforms found ith e upper coastal intertidal zone between land and brackish H2O. These are normally piquant dirt with the composing of peat and clay. Vegetation on these are largely of halophytic or salt tolerant as herbs, low bush or grasses.
Back Swamps or Basins:
These are drainage system features on landforms to modulate the rainwater and tidal H2O. It besides drains out H2O from depression. Smaller brook go through phases of transmutation and bring forth river channels to run out out H2O from depressions. Here the H2O bit by bit losingss it speed and suspends coarser deposits by the levee and carries finer atoms towards low lying swamps.
Ridges or Levees
During the fluxing flood of rivers with finer atoms creates ridges at the border of river channels through deposition of finer deposits. From the physiographic point of position the levees or ridges are the most raised lands of deltaic formation.
Tidal Brook:
Brooks are the tidal dendritic drainage system that are of about few centimetres to several metres in deepness and few 100 metres in breadth. These brooks form the complex web of wetlands as the smaller units to make watercourses and rivers with complex integrating procedures.
Sand Dunes:
Sand dunes are mound or ridge of sand that forms over a long period of clip by pin downing or windblown littorals via grasses or other obstructions.
Sandy beach
Beachs are the inclining zone of loose atoms such as sand, crushed rock, pebbles at the shoreline of ocean, sea, rivers etc. So the flaxen beach is found near the shoreline for illustration the flaxen beach at southern Dublar char.
Lithological Sequence Survey
Purposes and Aims
The Sundarbans is a tide and river dominated delta. Both the integrated rivers and channels of Ganges-Brahmaputra river and tidal activity of ocean are impacting the accumulation and erosional province of landforms. From the late Pleistocene period, the gradual deposit of rivers, tidal activities, mangrove flora every bit good as the tropical monsoon clime have been lending to the typical sedimentary beds in the Sundarbans.
The Aims and aims of lithological study in Sundarban:
- Study of Geomorphological landforms of the Sundarbans
- Lithological Stratification of the Sundarbans
- Designation of dirt features of strata
- Analysis of lithological strata to understand the formation features
Study Area: Dublar Char
Dublar Char was the survey country where the lithological study was conducted. It is located at the utmost south-central portion of the Sundarban of Bangladesh between 21031a??00a??a??N to 21043a??00a??a??N latitude and 89031a??40a??a??E to 89036a??20a??a??E longitude. It is one of the island of Sundarban Sharankhola Range, under the East Forest Division of Bangladesh in Bagerhat territory. The entire country of Dublar Char is about 67.93sq kilometers with a boundary of 40.89km. The island is crisscrossed with figure of river channels numbering 7328052.32sq kilometer of country.
Methodology and Data Collection
The lithological study was carried out in two stages, one is instant observation and analysis and another is sample aggregation for research lab analysis. The entire process can be categorized in four stairss:
- Soil profile analysis in Match-stick technique
- Soil sample aggregation in Monolith
- Soil sample aggregation in Boring method with gage sampling station
- Analysis of dirt sample by Troels-Smith Scheme and Color designation with Munsell colour chart
Two dirt profiles have been studied in two distant sites and overall five drilling operation was conducted. Two of the drillings are operated at each profile sites, two of them are at near intertidal zone and the last 1 at intertidal zone of the southeast boundary of Dublar Char.
Stratigraphic Analysis
The deadening dirt samples are analyzed on the footing of Troels-Smith strategy ( 1955 ) . Following the stratification and features of dirt samples are given in tabular arraies and diagrams:
Consequences
From the instant observation and analysis at profile sites, three beds had been identified in site 1 ( fig- ) and four beds in site 2 ( fig- ) .
In instance of borehole samples, the stratification ( fig- ) shows that the dirt is chiefly clay dominated and the upper beds contains more or less all right littorals. The clay constituent additions with deepness. The colour of the dirt gets darker with deepness, in general from Grey brown to really dark Grey or black. The dirt beds are normally more or less Fe stained, moist and organic in many beds. The dirt found at 1m above and below average sea degree contained wood fragments of different sizes. Fresh herbaceous rootlets are besides found in upper beds.
Except other beds, three really typical beds have been identified in Dublar Char dirt strata. Two of them are peat beds that are found at 45 to 64cm height at near-intertidal zone and at -210 to -232.24 cm height in intertidal zone. Though the peat beds were present in those samples, the inland samples show no bed of peat. The deposit form indicates that there can be peat beds at inland in higher deepness. Besides the peat bed, a flaxen dirt bed exists at 65 to -94cm height which is an exclusion in the deposit sequence in Dublar Char.
